In the oil and gas industry, numerous techniques of determining the geological structure of formations in the earth have been developed. One of the most common techniques is called "vertical seismic profiling" (VSP). This method is used to obtain information concerning a formation of interest. Typically, this technique requires the lowering of a geophone into a wellbore while providing at the surface seismic energy sources such as a dynamite charge. The explosion of the dynamite charge creates a seismic wave which is detected by the geophone located in the wellbore.
Another technique that is used is reverse vertical seismic profiling (RVSP) where numerous geophones are located on the surface and a dynamite charge or other energy source is activated in the borehole. By using these types of well known methods, subterranean formations can be probed with seismic waves to delineate their structure and physical properties. These conventional VSP and RVSP methods typically operate using seismic waves in the frequency range of 10-200 hertz and are capable of resolving structural details having dimensions of about 15-20 meters or larger.
In order to obtain improved resolution of underground geologic structures, the VSP and RVSP methods must be capable of operating at seismic wave frequencies up to about one order of magnitude higher than the conventional methods. However, one of the common problems with conventional seismic detectors is that their low-frequency design attributes are inappropriate for high-frequency measurements up to 2,000 hertz. To detect frequencies over the entire range, the seismic detector needs to be approximately the same density as the formation being measured and must be held in place in such a manner that it appears to be part of the formation itself. Some examples of seismic borehole devices can be found in U.S. Pat. No. 4,845,990 to Kitzinger, U.S. Pat. No. 4,715,470 to Paulsson, and U.S. Pat. No. 4,702,343 to Paulsson. However, the devices as described in each of these patents are not suitable for operating over a frequency range of 200 to 2,000 hertz because the devices are considerably heavier than the mass density of the geological formations in which they must operate. For such a heavy device to be held in position by the formation, the borehole clamping mechanism causes concentrated stresses at the points of contact with the geological formation. Concentrated stresses impose borehole resonance effects which distort the signal being received.
It is also important to know the azimuthal orientation of the seismic waves being detected. None of the patents described have an azimuthal orientation means for determining the direction from which the signal may originate. Directional orientation has been used in other types of downhole devices such as shown in U.S. Pat. No. 4,923,030 to Meynier and U.S. Pat. No. 3,614,891 to Nolte. However, neither of these devices pertain to probes intended to be oriented first and then clamped into place in a borehole for seismic detection.
High resolution seismic measurements have been used in characterizing the physical properties of rock, geological structure, and fluid migration pathways in oil and gas reservoirs by using seismic source and detector devices capable of operating over the frequency range of 200 to 2,000 hertz. Several seismic source transducers designed for borehole operation are available for this high resolution application. For example, borehole hydrophones have been used as detectors because of their useable frequency response up to 2,000 hertz and higher. Hydrophones generally provide a wideband frequency response to the incident seismic waves, but, because of the indirect nature of their response (i.e. scalar-pressure response to motional displacements of the borehole wall), the output signal does not contain information on the vector seismic displacement of the formation the borehole.
An important requirement in seismic characterization of oil and gas reservoirs is to detect and resolve the direction of the propagating wavefronts of both compressional and shear waves arriving at the detector. In the case of shear waves, the polarization orientation of the detected shear waves must be resolved. Hydrophone detectors, although indirectly responsive to incident shear waves, are not capable of providing a quantitative response indicating the vector of the seismic wavefront or the polarization of the shear wave motional displacements. Therefore, a seismic detector capable of accurately and directly sensing the three-dimensional motional displacement of the borehole wall is needed. The requirements for this type of seismic detector may be met by the use of three vector type sensors such as geophones or accelerometers aligned along orthogonal axes and mounted in a rigid borehole probe which can be temporarily clamped into place. This will allow the detector to be responsive to the three-dimensional seismic motions of the drilled formation.
Previous forms of borehole wall-locking seismic detector probes have consisted of a combination of one or more seismic detector elements; typically velocity sensitive geophones combined with one or more clamping arms which serve to hold the probe against the borehole wall for measurements. The clamping arms may be electrically or hydraulically driven. These types of probes have been relatively heavy, consisting of a pressure-resistant housing and a high-force mechanical clamping mechanism. In these prior devices, their inertial mass and self-contained compliant locking arm comprise a resonant structure that has prevented their use as three-component seismic detectors at frequencies above about 300 hertz. This creates spurious transverse resonances at frequencies above about 300 hertz, depending upon the particular design characteristics of the detector under consideration. Moreover, the geophone detector elements widely used for such seismic measurements typically exhibit a transverse (cross-axis) sensitivity in the range of 10-15 percent of the primary axis sensitivity.
Smaller and lighter weight borehole wall-locking seismic detectors are needed to overcome these limitations and to provide three-component seismic measurements for high resolution applications primarily in the frequency range of 200 to 2,000 hertz.